1. Field of the Invention
The present invention relates to analog electronic circuits and systems. More specifically, the present invention relates to power amplifiers and the like.
2. Description of the Related Art
In the fabrication of electronic circuits, particularly monolithic integrated circuits, there is often a need for complementary output devices. Complementary output devices are transistor devices fabricated in accordance with complementary technologies such as n-channel and p-channel devices in metal-oxide semiconductor field effect transistor (MOSFET) technology and npn and pnp transistors in bipolar technology. The use of complementary output devices substantially simplifies the biasing required for the output devices. This is particularly important where circuit size, complexity and power output are significant design considerations such as monolithic power amplifiers.
Unfortunately, it is very difficult to fabricate monolithic n-channel and p-channel devices having similar operating characteristics on the same substrate. The technology favors n-channel devices as these can be made smaller for the same voltage and current ratings. (In bipolar technology, the same principles generally hold for pnp as compared to npn technologies.)
Accordingly, in the design of circuits where complementary power devices would be of benefit, conventional wisdom is to wrap a small p-channel device around a larger n-channel device. The combination in many respects operates like a large n-channel device. Unfortunately, it is very difficult to produce a stable bias in such arrangements using conventional biasing techniques. For example, in many applications, such as monolithic power operational amplifier integrated circuits, such conventional bias techniques typically produce asymmetrical output signals, unstable quiescent current over temperature, and high output impedance, as well as distortion, overload recovery problems, overshoot and other nonlinearities.
Thus, there is a need in the art for an improved compound output stage. More specifically, there is a need in the art for a system and technique for biasing the output stage of a monolithic power amplifier in which a small p-channel device may be used to drive a larger n-channel device tied to a source of negative supply to provide a symmetrical alternating current (AC) output with control of the quiescent current in same.